Brake Bleed Puzzle

[Dejathoris]

After reading a lot of posts on the subject, I still was not satisfied I found the smoking gun solution to my problem, so hope someone here can help. I purchased a 1972 911 Christmas Eve, and it's been on jack stands ever since while I go over it with a fine-tooth comb. It has '84-'89 Carrera front calipers with new pads, and '75 rear calipers (near new) and new pads. Brake hoses look in good condition and are dated 2/99 in front and 3/00 for rears, no cracks or sponginess. Starting with furthest calipers and working to nearest, the rear calipers bled just fine, i.e. good fluid flow. The fronts, however, showed markedly little and slow fluid coming from the bleeders, but we got all the new fluid through. Here's the puzzle: if I immediately pressed the brake pedal after closing the bleeder on either of the front calipers (after pressing the brake pedal 3 times with the bleeder open), the pedal had little resistance, i.e. it was almost like the bleeder was still open, however, if I waited ten seconds after closing the bleeder and then pressed the pedal, the pedal was firm. I think that the seal for the front circuit in the master cylinder is bad, and need to replace the MC. What do you think?

Thanks.

[Bill Verburg]

Sounds likely, pressure bleeding is the preferred method. When older m/c are bled w/ the pedal the seals are often damaged from running over unfamiliar territory so to speak.

[Dejathoris]

Thanks for your response. If using the pedal method could do this, I have to wonder about the engineering. However, I am glad I budgeted for unknowns when I bought this car, as I was well aware these can be like boats, i.e. something that you can throw a lot of $ at. Anyone else concur the M/C is bad or have a different idea? On a related topic, anyone have an informed opinion about rubber brake hose (DOT) life. I've read only in one place on this forum 10 years, and heard from a racer 15 years max before replacing for a non or rarely tracked 911.

[Steve@Rennsport]

Bill is spot-on: pressure bleeding is the best method to flush and bleed the system as it prevents MC damage.

If your car has the original MC, its very likely that its ruined from pumping the pedal to the very end of its travel and after 30+ years, it should be replaced, anyway. In general, I don't like to wait until those actually fail to replace them,...

I replace OEM rubber brake hoses every 10 years. Less, if its a garage queen.

[jonesb930]

What is the preferred or best pressure system to accomplish this?

[janz]

I'm no expert but have bled a lot of brakes.
"(after pressing the brake pedal 3 times with the bleeder open)"
I have never used that way to bleed. Close bleeder then pump a few times and hold down brake pedal and open bleeder enough so pedal goes down try not to smash it to the floor board. Once almost all the way down close bleeder and repeat until pedal is firm. Maybe you just wrote it wrong but after reading it 3 times if this is actuall what you did you'll never get all the air out and yes you can damage the MC that way.

[CCM911]

Motive Power Bleeder, which our host sells. I will never go back to pumping the pedal again.

[1990C4S]

I have had good luck with gravity. Twenty minutes with the bleeder open. I believe Noah sold me on this. And he's a lot smarter than me.

[Dejathoris]

Thanks all for your responses. I guess I could have written it better, but the way I did it is exactly as Janz wrote. Given the upgrades that were on this car (turbo tie rods, f/g front and rear bumpers, f/g hood, adj. camber plates, f/g fender flares, lowered, Carrera front brakes, A/c ripped out, Pagid black pads, and what looks like electronics for plugging in at the track), my guess is this car saw a bit of the track. Can't tell if the M/C is original, but I will order a new one tomorrow with new hoses. Can't wait to fire her up. I had a list of 61 items to do, make that 63 with the M/C and hoses, and have 45 done. I just need to stop adding TTD!

[RWebb]

you can make your own system from a $10 garden sprayer, if you don't want to pay the $$ for the Motive

use a quality brake fluid

and you are right to avoid buying a used m/c - the savings is not worth the risk of death

sounds like the car is new to you - have you seen the Long Term & Commonly Neglected maintenance thread?

[Dejathoris]

I did awhile ago but never printed it out. Thanks for reminding me. A lot of stuff on the list I have done, such as pedal bushings, shifter bushings, all rubber fuel lines, all filters (oil, fuel, air) , all fluids (oil, brake, trans), all vacuum hoses, all light bulbs (except headlights), front drop link bushings, any other hard rubber, such as the windshield rubber, with the M/C, rubber brake lines, and rear sway bar bushings. It seems there is always something, but my wife is understanding and I am big on preventative maintenance and want to ensure the basics, specially safety issues, are done before I start driving it. This is specially w/r/t items that are suspect because I do not have too many previous maintenance records, even though it was owned by a local Porsche mechanic two years ago.

[911pcars]

I'm an advocate of atmospheric pressure.

Sherwood

[1990C4S]

I said it was Noah that told me gravity/atmospheric pressure. I guess it was Dr. Sherwood.

Either way, still smarter than me.

[911pcars]

Me of the KISS way of life which precludes random hose separation and resultant wet BF t-shirt/shiny paint contests and wastage of not insignificantly costly BF.

Patience grasshopper. Be one with gravity.

Sherwood

[moneymanager]

I repeat this about every 6 months.

Many of you, especially those with early less complicated cars, can bleed your brakes easily in less than 30 minutes without an assistant, without spending more than $5 on equipment, and without causing any undue stess on the master cylinder. My 73 7 74 911s lack a hydraulic clutch so I don’t know if this procedure can be adapted for use with non-mechanical clutches.

You will need: brake fluid, six or seven feet of plastic tubing sized to fit snugly over the nipple on the brake caliper (mine uses 3/16” internal diameter tubing but check yours), and an appropriate small box wrench to open & close the bleed valve. (Sears has small 6 sided combo wrenches which are useful for recalcitrant cases; the bleed valves on my car are 7mm but some I believe are 8 or 9mm.)

Here’s the procedure: jack the car up, support it on jack stands or wood blocks, and pull the wheels so you can get at the calipers. Or do one wheel at a time if you want.
Loosen the brake fluid container cover in the trunk.
At the first wheel, fix the plastic tubing so it hangs straight down (from your garage roof or door or even a stick or spouse) to the caliper. Slide the box wrench over the nipple, slide the plastic tubing over the nipple, and open the nipple with the wrench.
Watch the fluid rise in the tubing until it reaches the height of the brake fluid reservoir in the car, maybe 15 inches. This will take 3-4 minutes. Tap the caliper a few times with a rubber hammer or block of wood if you want. Watch the tubing for bubbles, especially when you start. A flashlight held behind the tubing will reveal all. If the bubbles keep on coming after the level in the tube has risen 12-15 inches, close the nipple, remove the tubing, drain the fluid in it into a paper cup, and repeat. Assuming you are bubble free after 12-13 inches of fluid have drained, however, do this once; tighten the wrench, pull the tubing off while holding a cup to catch the fluid, and move on to the next wheel.
Do all four wheels, keeping an eye on the fluid container and replenishing the brake fluid when it's down more than an inch.
You are done. Gravity has done all the work. You cannot damage your master cylinder with this technique, and you don’t need a helper (if you have a garage or a tree or can rig a stick to hang the tubing from.) The process should take you a maximum of 5 minutes per wheel once the car is up and the wheels are off.
Forget the idea that there are bubbles all thru your lines and that pressure will somehow push them out but not the fluid. A pressure bleeder only moves any bubbles and fluid thru the lines more quickly. But they travel together. So gravity will do the job perfectly if time isn’t the object.
Forget also the notion that the pressure generated by your brake bleeding kit will somehow give you a harder pedal. You can’t increase the pressure inside the system any more than gravity allows; if you could the fluid would compress when you applied the brakes and reduce the effectiveness of the brakes.
This process works well because nearly all the bubbles generated by driving around and heating the brakes excessively are found at the very top of the caliper near the bleed valve…where they concentrate when generated by heat. (Any 5 year old child knows bubbles don’t move down in a liquid, so they don’t flow down to the bottom of the caliper then across the hard lines and back up to the master cylinder. If they did, they’d eventually escape from the master cylinder reservoir into the atmosphere anyway.)
Depending on how much fluid you withdraw each time, you will find that the fluid is fully replaced every 10-15 bleedings. I track my car 5-6 times per year; bleeding pretty much replaces all the fluid over a couple of years, as recommended by many experts.

[big911fan]

I agree with the gravity or pressure bleed methods described. If you must (or want) to pedal bleed your system, try this. Put a small block of wood under to brake pedal, just thick enough to be about half the distance from the pedal to the floor. This should prevent you from pushing the seals past their normal travel distances. It may take a few more pumps but it could save you damaged seals.